Method for removing ammonia from non-condensable gases of a pulp mill

Abstract

A method for removing ammonia (NH 3 ) from a gas produced in a pulp mill ( 100 ). The method comprises producing raw non-condensable gas comprising ammonia (NH 3 ) in the pulp mill ( 100 ) and transferring at least some of the raw non-condensable gas to a scrubber ( 200 ) containing aqueous scrubbing solution ( 130, 140 ). The method comprises adding a compound capable of decreasing a pH of the scrubbing solution to the scmbbing solution ( 130, 140 ) and in the scrubber ( 200 ), contacting the raw non-condensable gas with the scmbbing solution ( 130, 140 ), to react the ammonia (NH 3 ) of the raw non-condensable gas with the scmbbing solution to produce clean non-condensable gas and ammonium (NH 4 + ). A pulp mill comprising equipment for performing the method.

The invention claimed is: 
     
       1. A method for removing ammonia (NH 3 ) from a gas produced in a pulp mill, the method comprising:
 producing raw non-condensable gas comprising ammonia (NH 3 ) in the pulp mill, 
 transferring at least some of the raw non-condensable gas to a scrubber containing aqueous scrubbing solution, 
 in the scrubber, contacting the raw non-condensable gas with the scrubbing solution, to react the ammonia (NH 3 ) of the raw non-condensable gas with the scrubbing solution to produce clean non-condensable gas and foul condensate comprising ammonium (NH 4   + ), 
 adding a compound capable of decreasing a pH of the scrubbing solution to the scrubbing solution, 
 measuring a pH of the scrubbing solution, 
 controlling a pH of the scrubbing solution based on the measured pH value by controlling a flow of the compound capable of decreasing a pH of the scrubbing solution to the scrubber, 
 in the scrubber, spraying the scrubbing solution to form droplets of the scrubbing solution, 
 contacting the non-condensable gas comprising ammonia with the droplets of the scrubbing solution, 
 conveying the foul condensate from the scrubber to a stripper, 
 stripping the foul condensate to produce clean condensate, 
 conveying the clean condensate to a pulp bleaching area, 
 bleaching pulp in the pulp bleaching area using the clean condensate to produce bleached pulp and bleaching effluent, 
 conveying the bleaching effluent to a wastewater treatment area, and 
 purifying water in the wastewater treatment area using the bleaching effluent, 
 wherein:
 the bleaching effluent comprises ammonium (NH 4   + ), 
 the compound capable of decreasing a pH of the scrubbing solution comprises sulfuric acid (H 2 SO 4 ) and 
 a pH of the scrubbing solution is from 4.5 to 5.5. 
 
 
     
     
       2. The method of  claim 1 , further comprising the steps of:
 determining that the measured pH value exceeds a first threshold, and 
 adding acid to the scrubbing solution. 
 
     
     
       3. The method of  claim 2 , wherein a pH of the acid is less than 4. 
     
     
       4. The method of  claim 2 , wherein the first threshold is 5.5. 
     
     
       5. The method of  claim 1 , wherein:
 the raw non-condensable gas comprises at least 1000 ppm ammonia (NH 3 ), and 
 a content of ammonia of the raw non-condensable gas is higher than a content of ammonia of the clean non-condensable gas. 
 
     
     
       6. The method of  claim 5 , wherein a content of ammonia of the clean non-condensable gas is less than 1000 ppm. 
     
     
       7. The method of  claim 1 , wherein the raw non-condensable gas comprises at least a compound comprising Sulfur. 
     
     
       8. The method of  claim 7 , wherein the raw non-condensable gas comprises at least one of hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide. 
     
     
       9. The method of  claim 7 , wherein the raw non-condensable gas comprises at least 20000 ppm methyl mercaptan on dry basis. 
     
     
       10. The method of  claim 7 , wherein the raw non-condensable gas comprises at least 1000 ppm ammonia on dry basis and at least 20000 ppm methyl mercaptan on dry basis. 
     
     
       11. The method of  claim 1 , further comprising the steps of:
 conveying at least some of the clean non-condensable gas to a furnace, and 
 burning the at least some of the clean non-condensable gas in the furnace. 
 
     
     
       12. The method of  claim 11 , further comprising the step of burning, in addition to the clean non-condensable gas, also other fuel in the furnace. 
     
     
       13. The method of  claim 12 , further comprising the step of recovering heat from the furnace by using a heat exchanger. 
     
     
       14. The method of  claim 13 , further comprising the step of utilizing the recovered heat to operate a steam turbine. 
     
     
       15. The method of  claim 1 , wherein the water is purified without addition of other nitrogen-containing compound, the other nitrogen-containing compound being urea.